Add `bindAddress` parameter to new proxy POST requests

Alternate implementation to enhancement request #22 by @dpapworth.

With the [current implementation](#23), if a browser on the same host specified ["localhost", the proxy was unreachable](#39).

Instead, suggest `0.0.0.0` is the default bind address, but clients can specify an optional `POST` parameter, `bindAddress` to bind `ProxyServer` to a specific IP.